Television signal processing systems are known which produce a video signal which, when displayed by a display device, will produce a picture having a main image with an auxiliary image inserted within (picture-in-picture, or "PIP") or next to (picture out of picture, or "POP") the main image. The two displayed video images are typically associated with video signals derived from different video sources (e.g., one from a first tuner tuned to one station and one from a second tuner tuned to another station or from a video source such as a VCR or laser disk player).
In order to create the above-described PIP or POP effect the auxiliary image video signal generally is decoded (demodulated) into luminance information and color difference information and then re-encoded to match the main picture video signal. This re-encoding is particularly critical in the chrominance channel, where the saturation (level) and tint (phase) of the auxiliary image is modulated by (and therefore dependent on) the burst signal from the main signal in order to maintain the proper saturation and tint when the combined (main and auxiliary) signal is decoded by the main chrominance decoder. A prior art television system 100 including a modulated chrominance PIP system is shown in FIG. 1. A main luma/chroma separator 104 is coupled to a main video source 102 and produces separated main luminance YM and chrominance CM components. The main luminance YM and chrominance CM components are then coupled to an overlay switch 106. An auxiliary luma/chroma separator 110 is coupled to an auxiliary video source 108 and produces separated auxiliary luminance YA and chrominance CA components. The auxiliary chrominance component CA is demodulated using the auxiliary burst reference by chrominance demodulator 112 to form color difference signals R-Y and B-Y. The color difference signals R-Y and B-Y are then processed (e.g., compressed) by a PIP processor 114 and coupled to a chrominance modulator 176. Chrominance modulator 116 re-modulates the difference signals using the main color burst reference to form chrominance component signal CA'. The re-modulated chrominance component signal CA' and the auxiliary luminance signal YA are coupled to the overlay switch 106.
Overlay switch 106 combines the two sets of component video signals YM,CM and YA,CA' into a set of combined component video signals Y,C. The combined chrominance signal C is demodulated using the main burst reference by chrominance demodulator 118 to form combined color difference signals R-Y" and B-Y". The combined color difference signals R-Y" and B-Y", and the combined luminance signal Y is coupled to a matrix processor and driver circuit 120, which responsively produces display driver signals R, G and B to drive display unit 122.
In the above system, chroma demodulator 118 utilizes timing (synchronization) information and color reference (color burst) from the main luminance and chrominance components to process the combined luminance and chrominance signals because the timing and color reference information from the auxiliary luminance and chrominance components are removed during the auxiliary signal processing. Chroma demodulator 118 may include a "color killer" circuit for suppressing chroma artifacts (e.g., "confetti") in monochrome pictures. As such, if the color killer circuit determines that the (main) video signal does not contain a color burst then the circuit will responsively kill all chroma information.
Unfortunately, if the video signal used to produce a main image does not exist or is monochromatic and does not contain color information (or the information is noisy or otherwise unusable) then both the main and auxiliary picture will be displayed as black and white images, even if the auxiliary video signal included color information. For example, the main image video signal may not include a burst signal (e.g., black and white or monochrome signal) or the burst signal may be very small or distorted (e.g., noisy conditions, severe antenna or tuner tilt across the video band). These conditions can cause loss of color to the auxiliary image.